Methods for fabricating and etching porous silicon carbide structures

ABSTRACT

The present disclosure relates to methods of fabricating a porous structure, such as a porous silicon carbide structure. The methods can include a step of providing a structure to be rendered porous, and a step of providing an etching solution. The methods can also include a step of electrochemically etching the structure to produce pores through at least a region of the structure, resulting in the formation of a porous structure. The morphology of the porous structure can be controlled by one or more parameters of the electrochemical etching process, such as the strength of the etching solution and/or the applied voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/434,049, filed Jun. 6, 2019, for “METHODS FOR FABRICATING AND ETCHINGPOROUS SILICON CARBIDE STRUCTURES,” which is a continuation of PatentCooperation Treaty (PCT) Application No. PCT/US17/66584, filed Dec. 15,2017, for “METHODS FOR FABRICATING AND ETCHING POROUS SILICON CARBIDESTRUCTURES,” which claims the benefit of U.S. Provisional ApplicationNo. 62/435,605, filed Dec. 16, 2016, for “METHODS FOR FABRICATING ANDETCHING POROUS SILICON CARBIDE STRUCTURES,” each of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to methods of fabricating ormanufacturing porous structures, such as porous silicon carbidestructures. The methods can employ electrochemical etching to produce aporous structure having a controlled and/or selected morphology.

SUMMARY

The present disclosure relates to methods of fabricating ormanufacturing porous structures, such as silicon carbide structures. Themethods can employ electrochemical etching to produce a porous structurehaving a controlled and/or selected morphology. In some embodiments, themethods comprise a step of providing a structure to be rendered porous,a step of providing an etching solution, and a step of electrochemicallyetching the structure to produce pores through at least a region of thestructure.

The resulting porous structure can comprise a morphology that can beselected and/or controlled as desired. For example, variouscharacteristics of the morphology, such as the pore diameter, the porewall thickness, and the porosity, can be controlled by selecting,varying, and/or adjusting one or more fabrication parameters. Further,various characteristics of the morphology can be controlled selectivelyand/or independently as desired.

In various embodiments, a selected morphology can be achieved bycontrolling the propagation rate of the etch front as it advancesthrough the structure during the electrochemical etching process. Thepropagation rate can be controlled by selecting, varying and/oradjusting the concentration of the reducing agent and/or the appliedvoltage. The propagation rate can be inversely related or inverselyproportional to the porosity that is achieved.

In certain embodiments, the methods disclosed herein further provide forfabricating a porous structure having a substantially uniformmorphology. The methods can also provide for monitoring the poreformation during the electrochemical etching process.

In additional embodiments, the methods disclosed herein also provide foretching either side of a silicon carbide structure. For example, the“Si” and/or “C” face can be etched in accordance with the principlesdisclosed herein to create a porous structure having a substantiallyuniform morphology.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

The written disclosure herein describes illustrative embodiments thatare non-limiting and non-exhaustive. Reference is made to certain ofsuch illustrative embodiments that are depicted in the figures, inwhich:

FIGS. 1A-1C are cross-sectional scanning electron microscopy (SEM)images of a variety of porous silicon carbide structures fabricated inaccordance with an embodiment of the present disclosure.

FIGS. 2A-2C are SEM images of a porous silicon carbide structure, whereFIGS. 2A-2B are cross-sectional images and FIG. 2C is a planar image ofa surface of the porous silicon carbide structure.

FIGS. 3A-3C are SEM images of another porous silicon carbide structure,where FIGS. 3A-3B are cross-sectional images and FIG. 3C is a planarimage of a surface of the porous silicon carbide structure.

FIGS. 4A-4C are SEM images of another porous silicon carbide structure,where FIGS. 4A-4B are cross-sectional images and FIG. 4C is a planarimage of a surface of the porous silicon carbide structure.

FIG. 5A is a cross-sectional SEM image of a porous silicon carbidestructure, according to another embodiment of the present disclosure.

FIG. 5B is a plot of the anodization current vs. time corresponding tothe electrochemical etch process used in the fabrication of thestructure depicted in FIG. 5A.

FIG. 6A is a cross-sectional SEM image of a comparative porous siliconcarbide structure.

FIG. 6B is a plot of the anodization current vs. time corresponding tothe electrochemical etch process used in the fabrication of thecomparative structure depicted in FIG. 6A.

FIG. 7 is a cross-sectional SEM image of a porous silicon carbidestructure, according to another embodiment of the present disclosure.

FIGS. 8A-8B are cross-sectional SEM images of a porous silicon carbidestructure, according to another embodiment of the present disclosure.

FIG. 8C is a plot of the anodization current vs. time corresponding tothe electrochemical etch process used in the fabrication of thestructure depicted in FIGS. 8A-8B.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. The illustrativeembodiments described in the detailed description, drawings, and claimsare not meant to be limiting. Other embodiments may be utilized, andother changes may be made, without departing from the spirit or scope ofthe subject matter presented here.

Thus, the following detailed description of the embodiments of themethods of the disclosure is not intended to limit the scope of thedisclosure, as claimed, but is merely representative of possibleembodiments. In addition, the steps of a method do not necessarily needto be executed in any specific order, or even sequentially, nor do thesteps need to be executed only once.

Porous structures, such as porous silicon carbide structures having anordered morphology, possess physical, chemical, and electroniccharacteristics which may enable improvements in existing technologiesand provide functionalities in new technologies. For example, they maybe useful as sensors and filters for chemical and biological detectionand removal, catalysis, and electron-emissive surfaces (cold cathodes orfield emitters). However, porous structures can also require structuralcharacteristics which are application-dependent, such as uniformity andorder, and specific microstructural properties such as pore wallthickness and pore diameter. Further, in some applications, the porosityof a porous structure may need to be high, e.g., in excess of 90% (bymass or equivalent volume), for optimal performance of the end device.

For example, biological filters with sufficient thru-put can requirestructures that are macroscopic in area, where permeability is obtainedthrough high porosity. In such applications, control over pore diameterwould allow for selectivity in filtering.

As another example, catalytic structures can require specific activesurface areas. In such instances, control of effective surface area in aporous structure would allow for designed catalytic reaction rates.

In another example, field emitters may be fabricated from porousstructures (e.g., porous semiconductor structures). In suchapplications, uniformity of structure would allow for consistentperformance as the device wears during operation. Further, the fieldenhancement factor can be dependent on pore wall thickness and porediameter.

Silicon carbide (SiC) (e.g., a silicon carbide semiconductor), includingits polytypes (e.g., 3C, 4H, 6H polytypes), is a useful material formany of the above-mentioned applications. Silicon carbide is refractory,and comprises a high Young's modulus. Silicon carbide is also robust atextremely high temperatures, allowing for operation in manyenvironments. The properties of silicon carbide also permitself-cleaning through heat or chemical agents, enabling reusability whenused in biological and chemical sensors. Further, the 4H and 6Hpolytypes of silicon carbide are wide band gap semiconductors; in fieldemission applications this property can result in enhanced emissionthrough a work function lowered by electron affinity effects. Methods offabricating porous silicon carbide structures having a controlledmorphology is thus desirable and advantageous in many ways.

The present disclosure provides for such methods. In particular, thepresent disclosure provides for methods of fabricating or manufacturingporous structures, such as porous silicon carbide structures. Asdetailed below, the methods employ electrochemical etching, or“electroetching” or “anodization,” in a controlled manner to producepores through a region of a structure which results in the formation ofa porous structure. In certain embodiments, the method provides forfabricating a porous structure having a controlled, selected, and/ordesired morphology. As used herein, the term “morphology” can refer tovarious characteristics of the porous structure, including, but notlimited to, the pore diameter, the pore wall thickness, the porosity,and/or the pore arrangement. This morphology can be varied and/orcontrolled as desired by selecting, varying, and/or adjusting one ormore fabrication parameters of the electrochemical etching process. Forexample, the diameter of the pores, the thickness of the pore walls, theporosity of the porous structure, and the arrangement (e.g., order) ofthe pores, can each be selectively and/or independently controlled inaccordance with the principles disclosed herein.

In one embodiment, the method of fabrication comprises a step ofproviding a structure or material that is to be etched and/or renderedporous. This structure can also be referred to as a starting structureor starting material. Various types of structures and materials can berendered porous in accordance with the methods disclosed herein,including, but not limited to, semiconductor structures and materials(which can also be referred to as semiconductors or semiconductor wafersor slices), and silicon carbide structures and materials (including, forexample, silicon carbide semiconductors). Other types of structures andmaterials can also be rendered porous in accordance with the methodsdisclosed herein.

It is contemplated that the structures and materials to be renderedporous can have various shapes, sizes, and/or thicknesses. For example,the structures to be rendered porous can be substantially cube shaped,substantially cuboid shaped, substantially prism shaped (e.g., hexagonalprism shaped), or substantially cylindrical shaped. Cylindrical shapedstructures include, but are not limited to, traditional semiconductorwafers and/or slices or substrates. Structures having other shapes canalso be rendered porous as desired. Additionally, it is to be understoodthat the methods disclosed herein are not limited to structures of anyparticular shape, size, and/or thickness.

In some embodiments, the structure to be rendered porous comprises oneor more properties, each of which can be predefined or defined accordingto the structure. For example, in one embodiment, the structure to berendered porous comprises a defined electric carrier concentration.Illustrative carrier concentrations of a silicon carbide structure ofn-type doping can include, but are not limited to, concentrations fromabout 1.1×10¹⁸ cm⁻³ to about 1.8×10¹⁸ cm⁻³, or from about 1.2×10¹⁸ cm⁻³to about 1.4×10¹⁸ cm⁻³.

The structure to be rendered porous can also comprise a particularcrystalline structure or designation. For example, in some embodiments,the structure comprises a crystalline structure. And in certainembodiments, the structure comprises a polytype of silicon carbide.Illustrative polytypes of silicon carbide include, but are not limitedto, 3C, 4H, or 6H polytypes. Other polytypes or crystalline structuresof silicon carbide and other materials can also be used.

In certain embodiments, the properties of the structure or material tobe rendered porous can affect and/or otherwise impact theelectrochemical etching process and/or the selected fabricationparameters. For example, the carrier concentration can impact the porediameter and/or the etch propagation rate. In one embodiment, thecarrier concentration sets or defines the length scale which theelectric field lines are shielded during the etching process. In suchembodiments, the pore diameter decreases with increasing carrierconcentration. In another embodiment, the carrier concentration sets ordefines the electrical conductivity of the material. In suchembodiments, the anodization current (or etch rate) increases withincreasing carrier concentration. In some embodiments, the methodsdisclosed herein can account for the particular properties of thestructure or material. For example, the properties of the structure ormaterial (e.g., carrier concentration and/or crystalline structure) canbe measured, analyzed and/or otherwise known prior to theelectrochemical etching process. The fabrication parameters of theelectrochemical etching process (e.g., concentration of the reducingagent, use of a surfactant, and/or voltage, etc.) can then be selectedin accordance with the properties of the structure or material to berendered porous to achieve a selected or desired morphology.

The method of fabrication disclosed herein further comprises a step ofproviding an etching solution, which can also be referred to as anelectrolyte or anodizing solution. In some embodiments, the etchingsolution comprises an oxidizing agent (or oxidizer) and a reducing agent(or reducer). In particular embodiments, the etching solution furthercomprises one or more additional components or additives, including, butnot limited to, a surfactant. As further detailed below, the etchingsolution can work in conjunction with an applied voltage during theelectrochemical etching process to produce pores in and/or through aregion of the structure that is to be rendered porous.

As can be appreciated, the oxidizing agent can react with the materialof the structure to be etched (e.g., silicon carbide) to form a chemicaloxide (e.g., a silicon oxide) when voltage is applied during theelectrochemical etching process. For example, the oxidizing agent can bethe source of oxygen atoms that are driven into the material of thestructure to be etched and removed to form a chemical oxide when voltageis applied. Various types of oxidizing agents can be used, including,but not limited to, water, alcohols (e.g., ethanol, methanol, etc.),hydrogen peroxide, acetic acid or other acid-based oxidizing agents, ormixtures thereof. In a particular embodiment, the etching solutioncomprises water. For example, deionized water or distilled water can beused. As can be appreciated, deionized water may contain fewercontaminants, thereby minimizing and/or eliminating the formation ofundesirable by-products during the electrochemical etching process.Deionized water can also be less electrically conductive than distilledwater. In certain embodiments, water (e.g., deionized water) can be usedto produce a porous structure that comprises a relatively orderedarrangement of pores (e.g., an ordered porous structure) as compared toa porous structure that comprises relatively randomly arranged pores(e.g., a random porous structure).

The reducing agent can be used to remove the chemical oxide formed bythe oxidizing agent. Various types of reducing agents can be used,including, but not limited to, acidic reducing agents. In certainembodiments, the reducing agent comprises hydrofluoric acid. Variousstrengths and/or concentrations of hydrofluoric acid can be used. Forexample, the reducing agent can comprise hydrofluoric acid at aconcentration of from about 1% to about 50%, from about 1% to about 20%,from about 1% to about 15%, from about 1% to about 10%, from about 1% toabout 5%, or from about 2% to about 5% by volume (v/v %).

As previously mentioned, in certain embodiments, the etching solutionfurther comprises a surfactant. Surfactants can be used to reduce theviscosity of the etching solution. In some embodiments, reducing theviscosity with a surfactant can increase the etch rate and aid in theformation of a more controlled and uniform porous structure. Forexample, reducing the viscosity of the etching solution can allow forfaster diffusion of the oxidizing agent and/or reducing agent throughthe porous structure during the etching process. Exemplary surfactantsthat can be used include, but are not limited to, surfactants comprisingpolyoxyalkylene alkyl ether. In certain embodiments, surfactants thatare stable and do not readily decompose under electrochemical conditionsare used (e.g., surfactants that will not readily decompose whensubjected to solutions of hydrofluoric acid and/or high voltages).

The method of fabrication disclosed herein further comprises a step ofelectrochemically etching the structure to remove material and producepores through at least a region of the structure to form a porousstructure. As can be appreciated, the step of electrochemical etchingcan be performed using a variety of different techniques and equipment.For example, in one embodiment, the structure to be electrochemicallyetched can be placed in electrical connection with a positive electrodeor pole of a source of electrical current (e.g., a direct electricalcurrent). A negative electrode can also be provided and placed inelectrical communication with the electrical source. For example, aplatinum electrode can be used as a negative electrode for facilitatingelectrical connection to the circuit and can be immersed in the etchingsolution. The structure or portion thereof to be etched can then beimmersed in the etching solution. A voltage can then be applied acrossthe platinum electrode and the structure.

In certain embodiments disclosed herein, the step of electrochemicallyetching comprises applying a voltage to a selected region of thestructure that is to be etched or rendered porous. This voltage canproduce a current density through the selected region of the structure.Various voltages can be used, including, but not limited to, ranges fromabout 20 V to about 40 V, about 20 V to about 30 V, and/or about 20 V toabout 26 V. As further detailed below, the current density can bemonitored during the electrochemical etching process to provideinformation about the porous structure being formed. The current densitycan also be monitored such that it is kept above a threshold valuethroughout the electrochemical etching process.

The time period for which the electrochemical etching step is performedcan depend upon the desired characteristics of the structure to beetched, such as size, density, and configuration of the pores and voidsand the resulting porosity of the structure. Exemplary time periods forelectrochemical etching are from about 1 minute to about 8 hours, fromabout 5 minutes to about 4 hours, or from about 5 minutes to about 15minutes. Greater and/or lesser time periods can also be used.Additionally, it is contemplated that the etching step can be continuousor intermittent, and that the etching step may be performed upon one ormore faces (e.g., “Si” face and/or “C” face), or upon a portion or allsurfaces of the structure.

The electrochemical etching step can also be performed at varioustemperatures, including, but not limited to, ambient temperatures, whichare generally considered to be within a range of from about 65° F. (18°C.) to about 75° F. (24° C.). The electrochemical etching step can alsobe performed at a range of temperatures between the freezing and boilingpoints of the etching solution.

After subjecting the structure to the electrochemical etching step, thestructure is rendered porous. In certain embodiments, highly porousstructures can be obtained. For example, in some embodiments, theporosity of the porous structure is greater than about 60%, greater thanabout 65%, greater than about 70%, greater than about 75%, greater thanabout 80%, greater than about 85%, or greater than about 90%. In furtherembodiments, the porosity of the porous structure is from about 60% toabout 96%, from about 70% to about 80%, from about 80% to about 90%, orfrom about 70% to about 94%, as desired. In particular embodiments, theporosity can be characterized as nanoporous. The term “nanoporous” asused herein refers to a structure that includes numerous pore wallswhich have a size generally within the nanometer range.

As previously stated, in certain embodiments, the methods disclosedherein provide for fabricating a porous structure having a controlled,selected, and/or desired morphology. For example, the characteristics ofthe morphology of the porous structure can be controlled by selecting,varying, and/or adjusting one or more fabrication parameters of theelectrochemical etching process. The fabrication parameters can includethe components of the etching solution, such as the type and/orconcentration of the reducing agent, oxidizing agent, and optionally, asurfactant. The fabrication parameters can also include the voltageapplied during the electrochemical etching process.

In certain embodiments, the controlled morphology comprises one or moreof a controlled or selected pore diameter, pore wall thickness,porosity, pore arrangement (e.g., order), or combinations thereof. Forexample, in particular embodiments, the controlled morphology comprisesa controlled pore diameter. In such embodiments, the pore diameters ofthe porous structure can be selected and controlled as desired. Forexample, larger or smaller pores can be produced as desired. In someembodiments, the average pore diameter of the porous structure can beselected such that it is within a particular range, such as betweenabout 0.1 μm and about 3 μm. In further embodiments, the pore diametersof the porous structure can be selected such that they are substantiallyuniform throughout the porous region of the porous structure.

The pores of the porous structure can be selected such that they aresubstantially cylindrical in shape. The pores of the porous structurecan also be selected such that they are substantially dendritic inshape. Further, in certain embodiments, the walls of individual poresthroughout the porous structure can be etched or degraded such thatindividual pores may no longer be present or observable. In suchembodiments, the portions of the pore walls that remain can produce aporous structure that is needle-like in appearance, as the remainingportions of the pore walls may be disconnected from each other andappear as needles or substantially elongated segments.

In another embodiment, the controlled morphology comprises a controlledpore wall thickness. In such embodiments, the pore wall thickness of theporous structure can be selected and controlled as desired. For example,in one embodiment, the pore wall thickness of the porous structure canbe selected such that it is between about 2 nm and about 200 nm. Otherpore wall thicknesses can also be achieved as desired. In furtherembodiments, the pore wall thicknesses of the porous structure can beselected such that they are substantially uniform throughout the porousregion of the porous structure.

In another embodiment the controlled morphology comprises a controlledporosity. In such embodiments, the porosity of the porous structure canbe selected and controlled as desired. For example, in some embodiments,the porosity of the porous structure is selected such that it is greaterthan about 60%, greater than about 65%, greater than about 70%, greaterthan about 75%, greater than about 80%, greater than about 85%, orgreater than about 90%. In further embodiments, the porosity of theporous structure is selected such that it is from about 60% to about96%, from about 70% to about 80%, from about 80% to about 90%, or fromabout 70% to about 94%. Other porosities can also be achieved asdesired. In further embodiments, the controlled morphology comprises aselected material density.

In another embodiment the controlled morphology comprises a controlledpore arrangement. In such embodiments, the pore arrangement of theporous structure can be selected to be ordered as compared to morerandomly distributed. For example, in some embodiments, the pores can besubstantially aligned and/or parallel with one another. The pores canalso be ordered (e.g., physically ordered) and/or arranged in asubstantially uniform manner. For example, the pores can be evenlydistributed and/or spaced from one another throughout the porous region.The thicknesses of the pore walls and the pore diameters can also besubstantially similar and/or uniform throughout the porous region.

The ordered structure can comprise cylindrically-shaped pores. Infurther embodiments, the controlled morphology comprises an orderedstructure comprising dendritic-shaped pores. For example, in particularembodiments, lower voltages can be used to produce a porous structurecomprising a dendritic ordered structure. In other embodiments, thecontrolled morphology comprises a needle-like structure. For example, inparticular embodiments, higher voltages can be used to produce a porousstructure comprising a needle-like structure, such as the needle-likestructures previously discussed.

In further embodiments, the controlled morphology comprises asubstantially uniform morphology or porous structure. For example, themorphology can be selected such that it is substantially uniformthroughout the porous region of the porous structure. Further, themethods disclosed herein also provide for achieving a substantiallyuniform morphology in relatively thick structures. For example, in someembodiments, a substantially uniform morphology can be obtained instructures having a thickness of about 10 μm, about 20 μm, or greater.For example, in some embodiments, a substantially uniform morphology canbe achieved under constant etching conditions or parameters instructures having thicknesses of 20 μm or greater. In furtherembodiments, a substantially uniform morphology can be achieved throughcontrolled variation of the etching parameters. For example, theconcentration of the chemical constituents (e.g., the concentration ofthe reducing agent) can be varied during the electrochemical etchingprocess to achieve a substantially uniform morphology. As furtherdetailed below, pore formation can be monitored throughout theelectrochemical etching process, aiding in the controlled variation ofthe etching parameters.

Control over the morphology can be achieved in various ways. Forexample, in some embodiments, a selected morphology can be achieved bycontrolling one or more of the etching parameters of the electrochemicaletching process. For example, a selected morphology can be achieved bycontrolling the propagation rate of the etch front as it advancesthrough the structure to be rendered porous. The term “etch front” canrefer to the region of the structure that is being etched by the etchingsolution during the electrochemical etching process. As the etch frontadvances through the structure, pores are produced. The propagation rate(etching rate) can be manifested by the anodization current, and candecrease monotonically with time as the etch propagates. In someinstances, this decrease can be attributed to the diffusion of thechemical constituents through the pores becoming more difficult due tothe confined geometry of the pore structure. As the interfacial chemicalconstituents deplete, a threshold level can be reached which isdependent on the specific etch conditions. At this threshold level, theslope of the anodization current vs. time curve changes, reflecting adiscontinuous change in the removal rate with time. This signal ofdiscontinuity also indicates a change in morphology.

As can be appreciated, during the etching process, the etch front is atwo-dimensional plane that propagates into the structure (e.g., thesemiconductor wafer). As the etch front propagates inwards, thestructure can form substantially cylindrically-shaped pores (ordendritically-shaped pore if desired, etc.), with the long axis alongthe direction of propagation and perpendicular to the two-dimensionalplane that comprises the etch front. In silicon carbide structures, theintrinsic in-plane etching rate is higher than the etching rate alongthe long axis of the pores. Thus, if the propagation rate is decreased,more material is removed in-plane allowing for thinner pore walls andhigher porosity, for a fixed voltage (pore diameter).

In some embodiments, the propagation rate of the etch front can becontrolled by selecting, varying, and/or adjusting the concentration ofthe reducing agent (e.g., hydrofluoric acid) and/or the applied voltage(and/or optionally through the use of a surfactant). For example, insome embodiments, a selected concentration (or concentration range) ofthe reducing agent (e.g., hydrofluoric acid) can be used in conjunctionwith a selected applied voltage (or range of voltages) such that thepropagation rate of the etch front produces pores that are substantiallyuniform in size and shape. In further embodiments, increasing theconcentration of the reducing agent can increase the propagation rate ofthe etch front. Analogously, decreasing the concentration of thereducing agent can decrease the propagation rate of the etch front.

In particular embodiments, the concentration (or concentration range) ofthe reducing agent (e.g., hydrofluoric acid) and/or the applied voltage(or range of applied voltages) are selected such that the porosity ofthe porous structure is inversely related or inversely proportional tothe propagation rate of the etch front. For example, at high propagationrates (high anodization currents), the etch front moves rapidly into thematerial, with reduced lateral (spreading) etching in the pores.Consequently, less material is removed per etched length along thedirection of the pores and a denser structure is formed having adecreased porosity. Analogously, as the propagation rate decreases, morematerial is removed resulting in an increased porosity.

In certain embodiments, the concentration (or concentration range) ofthe reducing agent (e.g., hydrofluoric acid) and/or the applied voltage(or range of applied voltages) are selected such that the porosity ofthe porous structure is inversely related or inversely proportional tothe concentration of the reducing agent (e.g., hydrofluoric acid). Forexample, as the concentration or strength of the reducing agent (e.g.,hydrofluoric acid) increases, less material is removed per etched lengthalong the direction of the pores and a denser structure is formed havinga decreased porosity. Analogously, as the concentration or strengthdecreases, more material is removed resulting in an increased porosity.

The pore diameter can also be controlled by selecting, varying, and/oradjusting the applied voltage. For example, in some embodiments,increasing the voltage also increases the average pore diameter formedin the porous structure. Analogously, decreasing voltage can decreasethe average pore diameter formed in the porous structure.

In some embodiments, a selected combination of concentrations ofreducing agent and voltages can be used. For example, the selectedconcentration of the reducing agent (e.g., hydrofluoric acid) can befrom about 1% to about 50%, from about 1% to about 20%, from about 1% toabout 15%, from about 1% to about 10%, from about 1% to about 5%, orfrom about 2% to about 5%, by volume, and the selected voltage can befrom about 20 V to about 40 V, from about 20 V to about 30 V, or fromabout 20 V to about 26 V. Further, in certain embodiments, the selectedconcentration of the reducing agent (e.g., hydrofluoric acid) and/or theselected voltage are dependent upon one or more properties of thestructure to be etched or rendered porous. For example, theconcentration and/or voltage can be selected to correspond to astructure having a known or predetermined carrier concentration and/orcrystal structure such that a selected morphology can be achieved.

As previously mentioned, in some embodiments, the propagation rate ofthe etch front can be monitored to ensure a substantially constantlydecreasing etch rate and/or substantially uniform pore formation. Forexample, the current density across the structure to be rendered porouscan be monitored to ensure a substantially constantly decreasing etchrate. For example, in the anodization current vs. time plot, the valueof the current at any specific time is directly related to the removalrate of the etching process. A constant slope of this plot indicates aconstantly decreasing removal rate of the material, resulting insubstantially uniform pore formation. A change in this slope indicates achange in the rate of decrease of the etch rate, also signaling a changein pore formation or morphology. In some embodiments, monitoring theanodization slope can allow for adjustments to be made during theetching process, such as altering the strength of the chemicalconstituents (e.g., hydrofluoric acid), to provide for a constant slopein the anodization current vs. time plot, and substantially uniform poreformation.

In another embodiment of the present disclosure, the methods provide forminimization and/or elimination of a denser surface layer in the porousstructure. For example, in some traditional etching processes, themorphology of the porous structure differs at the surface of the porousstructure. For example, in some instances the porous structure has ahigher density at the surface. By controlling one or more of the etchingparameters, a denser surface layer can be avoided or minimized. Forexample, in some embodiments, the background interfacial dielectricconstant of the etching solution can be altered, for example, throughthe use of specific oxidizers and/or surfactants, or alternativelythrough selective variation of the applied voltage over time. The densersurface layer can also be removed via reactive ion etching if desired.

In yet another embodiment, the present disclosure provides for methodsof etching from either the “Si” face or “C” face of the silicon carbidestructure. As can be appreciated, silicon carbide is a piezoelectricmaterial. As a piezoelectric material, the silicon carbide structure canexhibit an internal electric field that is created in the presence ofstress or strain. In silicon carbide, intrinsic stress in the structurecan result in an internal electric field which leads to differences inthe chemical and physical properties of the two surfaces orthogonal tothe field. These two surfaces can be referred to as the “Si” and “C”faces.

The different properties of the “Si” face and the “C” face can affectthe electrochemical etching process. For example, on the “C” face, theinternal electric field can enhance the oxidation during theelectrochemical etching process, as opposed to the “Si” face where theoxidation rate can be reduced. As such, faster etching and more orderedand uniform structures were traditionally obtained when etching from the“C” face. However, by controlling one or more of the etching parameters,the methods disclosed herein are capable of being used to obtain ahighly porous, ordered, and substantially uniform structure when etchingfrom (e.g., beginning from) either the “Si” face or the “C” face of asilicon carbide structure.

In various embodiments, the methods disclosed herein also comprise animaging step to aid in qualifying and/or quantifying the morphology ofthe porous structure. For example, the method can comprise a step ofimaging the porous structure, for example, using a scanning electronmicroscope. Various characteristics of the morphology can then beassessed, including, but not limited to, the pore wall thickness,average pore diameter, porosity, and etch depth. Other methods known inthe art can also be used to assess the characteristics of themorphology.

For example, in some embodiments, the porosity can be measured usingmass measurement techniques. In such embodiments, the porosity can bemeasured by comparing the mass of the structure before and after it hasbeen rendered porous. The porosity can also be measured by analyzing thecurrent and/or charge used during the electrochemical etching process.For example, the area under the anodization current vs. time plot canyield the total charge used in the process. The charge is proportionalto the number of atoms removed, and is also a measure of the massremoved, and can be used to determine the porosity of the structure.

EXAMPLES

The following examples are illustrative of embodiments of the presentdisclosure, as described above, and are not meant to be limiting in anyway.

Example 1

Three silicon carbide structures (Samples A-C) were electrochemicallyetched in accordance with the principles disclosed herein to demonstratethe variation in porosity and pore wall thickness achieved throughcontrol of the propagation rate of the etch front. Each of the siliconcarbide structures was substantially the same size, shape, andthickness, and the electrochemical etching processes applied to each ofthe structures was substantially the same. The voltage applied duringthe electrochemical etching process for each of the samples wasapproximately 20 V. The only variable in the electrochemical process wasthe strength or concentration of the reducing agent (hydrofluoric acid).The etching solutions used for each of the samples are shown in Table 1.

TABLE 1 Sample No. Etching Solution (v/v %) Sample A 4% Hydrofluoricacid (Hydrofluoric acid, 49% (aqueous solution)) 30% Ethanol Balancedeionized water Sample B 6% Hydrofluoric acid (Hydrofluoric acid, 49%(aqueous solution)) 30% Ethanol Balance deionized water Sample C 9%Hydrofluoric acid (Hydrofluoric acid, 49% (aqueous solution)) 30%Ethanol Balance deionized water

After the samples were etched, the charge used in the electrochemicaletching process was analyzed and the porosity of the samples wasdetermined. The porosities of each of the samples is shown in Table 2.

TABLE 2 Sample No. Charge (C/μm) Porosity (%) Sample A 0.56 83% Sample B0.55 82% Sample C 0.52 79%

As shown in Tables 1-2, the porosity was inversely related or inverselyproportional to the strength or concentration of the hydrofluoric acidreducing agent. As the concentration of the hydrofluoric acid wasincreased, the porosity decreased.

The inverse relationship between the concentration and the porosity canalso be observed in SEM images that were taken of the samples. FIG. 1Ais a cross-sectional SEM image of Sample A, FIG. 1B is a cross-sectionalSEM image of Sample B, and FIG. 1C is a cross-sectional SEM image ofSample C. As shown therein, the pore wall thickness increased as theconcentration of the hydrofluoric acid was increased, resulting in adecreased porosity and increased density.

As further shown in FIGS. 1A-1C, the morphology in each of theindividual samples was substantially uniform. For example, the diameterof the pores and pore wall thickness remains substantially constantthroughout the porous structures. The pores are also ordered in each ofthe samples.

Example 2

Three more silicon carbide structures were electrochemically etched inaccordance with the principles disclosed herein to demonstrate thevariation in porosity and pore diameter achieved through control of thevoltage applied during the electrochemical etching process. Each of thesilicon carbide structures was substantially the same size, shape, andthickness, and the concentration of the hydrofluoric acid reducing agentremained the same. The etching solution used for each of the samples wasthe same, and is shown in Table 3.

TABLE 3 Sample No. Etching Solution (v/v %) Samples D-F 4% Hydrofluoricacid (Hydrofluoric acid, 49% (aqueous solution)) 0.5% PolyoxyalkyleneAlkyl Ether Surfactant Balance deionized water

The only variable in the electrochemical process was the magnitude ofthe applied voltage, which was varied between 20 V and 24 V, as shown inTable 4. After the samples were etched, the charge used in theelectrochemical etching process was analyzed and the porosity of thesamples was determined. The porosities of each of the samples is shownin Table 4.

TABLE 4 Sample No. Voltage (V) Charge (C/μm) Porosity (%) Sample D 200.56 83% Sample E 22 0.60 86% Sample F 24 0.71 93%

As shown in Table 4, the porosity was directly related to the magnitudeof the applied voltage. As the voltage increased, the porosity alsoincreased.

The relationship between the applied voltage and the pore diameter andporosity can also be observed in SEM images that were taken of thesamples. FIGS. 2A-2C are SEM images of Sample D, FIGS. 3A-3C are SEMimages of Sample E, and FIGS. 4A-4C are SEM images of Sample F. Morespecifically, FIGS. 2A-2B are cross-sectional SEM images, and FIG. 2C isa surface SEM image of Sample D; FIGS. 3A-3B are cross-sectional SEMimages, and FIG. 3C is a surface SEM image of Sample E; and FIGS. 4A-4Bare cross-sectional SEM images, and FIG. 4C is a surface SEM image ofSample F. As shown therein, the pore diameter and porosity increasedwith increased voltage. Like porosity, the pore diameter was directlyrelated to the voltage applied. Higher voltages produced a largeraverage pore diameter.

As further shown in FIGS. 2A-2B, 3A-3B, and 4A-4B, the morphology ineach of the individual samples was substantially uniform. For example,the diameter of the pores and pore wall thickness remains substantiallyconstant throughout the porous structures. The pores are also ordered ineach of the samples.

Example 3

Another silicon carbide structure was electrochemically etched inaccordance with the principles disclosed herein to create a poroussilicon carbide structure (Sample G) having a substantially uniformmorphology. The etching solution used is provided in Table 5, and thevoltage used in the electrochemical etching process was approximately 24V.

TABLE 5 Sample No. Etching Solution (v/v %) Sample G 3.5% Hydrofluoricacid (Hydrofluoric acid, 49% (aqueous solution)) 0.5% PolyoxyalkyleneAlkyl Ether Surfactant Balance deionized water

A cross-sectional SEM image of the resulting porous structure (Sample G)is shown in FIG. 5A. As shown in FIG. 5A, the morphology of the poroussilicon carbide structure was substantially uniform throughout theporous region. For example, the diameter of the pores and the pore wallthickness are substantially the same throughout the porous region. Thepores are also ordered.

The controlled and uniform porosity of Sample G are also evidenced bythe anodization current vs. time plot shown in FIG. 5B. As shown in FIG.5B, there is an initiation time of approximately 50 seconds after whichthe current decreases at a substantially constant rate over time,indicating that the pore formation is substantially uniform throughoutthe electrochemical etching process.

Example 4

A silicon carbide structure was electrochemically etched as a comparisonto the silicon carbide structure discussed in Example 3. The comparativesilicon carbide structure (Sample H) comprised a non-uniform morphology,as shown in a cross-sectional SEM image depicted in FIG. 6A. As shown inFIG. 6A, the porosity distinctly changes from the upper portion to thebottom portion of the structure. The non-uniform porosity is alsoevidenced by the anodization current vs. time plot shown in FIG. 6B. Asshown in FIG. 6B, the slope of the anodization current changes atapproximately 150 seconds, indicating a change in morphology.

In certain embodiments discussed herein, the fabrication parameters canbe modified or adjusted when a change in slope is observed. For example,the strength or concentration of the reducing agent could have beenincreased or adjusted to better control the morphology and uniformitythereof.

Example 5

Another silicon carbide structure was electrochemically etched inaccordance with the principles disclosed herein to create a poroussilicon carbide structure (Sample I) having a substantially uniformdensity throughout the porous region. In particular, the fabricationparameters were selected and/or controlled such that the porousstructure did not exhibit a significantly denser layer at the uppersurface. The etching solution used is provided in Table 6, and thevoltage used in the electrochemical etching process was approximately 20V.

TABLE 6 Sample No. Etching Solution (% v/v) Sample I 4% Hydrofluoricacid (Hydrofluoric acid, 49% (aqueous solution)) 0.5% PolyoxyalkyleneAlkyl Ether Surfactant Balance deionized water

A cross-sectional SEM image of the resulting porous structure (Sample I)is shown in FIG. 7 . As shown in FIG. 7 , the morphology of the poroussilicon carbide structure was substantially uniform throughout theporous region. For example, the diameter of the pores and the pore wallthickness are substantially the same throughout the porous region. Thepores are also ordered. A denser, upper surface layer was also minimizedand largely avoided, and the porosity was substantially constantthroughout the structure.

Example 6

Another silicon carbide structure was electrochemically etched inaccordance with the principles disclosed herein to demonstrateelectrochemical etching from the “Si” face of the silicon carbidestructure (Sample J). The etching solution used is provided in Table 7,and the voltage used in the electrochemical etching process wasapproximately 28 V.

TABLE 7 Sample No. Etching Solution (% v/v) Sample J 7.5% Hydrofluoricacid (Hydrofluoric acid, 49% (aqueous solution)) 19% Ethanol Balancedeionized water

A cross-sectional SEM image of the resulting porous structure (Sample J)is shown in FIGS. 8A-8B, where FIG. 8B is an enlarged image of a portionof FIG. 8A. As shown in FIGS. 8A-8B, a controlled porous silicon carbidestructure having a substantially uniform morphology was achieved whenetched from the “Si” face. The controlled and uniform morphology is alsoevidenced by the anodization current vs. time plot shown in FIG. 8C. Asshown in FIG. 8C, the current decreases at a substantially constant rateover time, indicating that the pore formation is substantially uniformthroughout the electrochemical etching process.

Reference throughout this specification to “an embodiment” or “theembodiment” means that a particular feature, structure, orcharacteristic described in connection with that embodiment is includedin at least one embodiment. Thus, the quoted phrases, or variationsthereof, as recited throughout this specification are not necessarilyall referring to the same embodiment.

Similarly, it should be appreciated that in the above description ofembodiments, various features are sometimes grouped together in a singleembodiment, figure, or description thereof for the purpose ofstreamlining the disclosure. This method of disclosure, however, is notto be interpreted as reflecting an intention that any claim require morefeatures than those expressly recited in that claim. Rather, as thefollowing claims reflect, inventive aspects lie in a combination offewer than all features of any single foregoing disclosed embodiment.

The claims following this written disclosure are hereby expresslyincorporated into the present written disclosure, with each claimstanding on its own as a separate embodiment. This disclosure includesall permutations of the independent claims with their dependent claims.Moreover, additional embodiments capable of derivation from theindependent and dependent claims that follow are also expresslyincorporated into the present written description.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A method of fabricating a porous silicon carbidestructure having a selected morphology, comprising: providing a siliconcarbide structure; providing an etching solution comprising a reducingagent and an oxidizing agent; electrochemically etching the siliconcarbide structure with the etching solution to produce pores through aregion of the silicon carbide structure to form a porous silicon carbidestructure, wherein electrochemically etching the silicon carbidestructure comprises applying a voltage to a surface of the siliconcarbide structure to produce a current through the region of the siliconcarbide structure; and controlling an etch propagation rate of theetching solution through the region of the silicon carbide structure toachieve the selected morphology by changing at least one fabricationparameter including one or more of concentration of reducing agent,voltage, and use of a surfactant, wherein the current decreases at asubstantially constant rate during the electrochemical etching step. 2.The method of claim 1, wherein the silicon carbide structure comprises a3C, 4H, or 6H polytype of silicon carbide.
 3. The method of claim 1,wherein the silicon carbide structure comprises a silicon carbidesemiconductor.
 4. The method of claim 1, wherein the silicon carbidestructure comprises a predefined electronic carrier concentration. 5.The method of claim 1, wherein the silicon carbide structure comprises apredefined crystalline designation.
 6. The method of claim 1, whereinthe oxidizing agent comprises water, an alcohol, hydrogen peroxide, or amixture thereof.
 7. The method of claim 1, wherein the reducing agentcomprises hydrofluoric acid.
 8. The method of claim 7, wherein theconcentration of the hydrofluoric acid is from about 1% to about 50%,from about 1% to about 20%, from about 1% to about 15%, from about 1% toabout 10%, from about 1% to about 5%, or from about 2% to about 5%, byvolume.
 9. The method of claim 7, wherein the etch propagation rate isdependent upon the concentration of the hydrofluoric acid.
 10. Themethod of claim 7, wherein the concentration of hydrofluoric acid isinversely proportional to a porosity of the porous silicon carbidestructure.
 11. The method of claim 1, wherein the etching solutionfurther comprises a surfactant.
 12. The method of claim 1, wherein thevoltage is from about 20 V to about 40 V, from about 20 V to about 30 V,or from about 20 V to about 26 V.
 13. The method of claim 1, wherein thecurrent is proportional to a removal rate of the material.
 14. Themethod of claim 1, wherein the selected morphology comprises at leastone of a selected average pore diameter, a selected pore wall thickness,or a selected porosity.
 15. The method of claim 1, wherein the selectedmorphology comprises an average pore wall thickness of between about 2nm and about 200 nm.
 16. The method of claim 1, wherein the selectedmorphology comprises an average pore diameter of between about 0.1 μmand about 3 μm.
 17. The method of claim 1, wherein the selectedmorphology comprises an average pore diameter of between about 0.1 μmand about 3 μm, and an average pore wall thickness of between about 2 nmand about 200 nm.
 18. The method of claim 1, wherein the selectedmorphology comprises a porosity of from about 60% to about 96%.
 19. Themethod of claim 1, wherein the selected morphology comprises asubstantially uniform material density.